Psychomotor stimulant drugs of abuse cause brain damage that is dependent on elevated body temperature. We have begun to examine brain temperature in association with self-administered drugs and other behaviors involving emotional arousal. We find that brain temperature is elevated 1-2? by the stress of handling and transport to the test cage, by mildly arousing tail pinch, and by exposure to another rat or a mouse. We find similar elevations during intravenous self-administration of heroin and cocaine. In each case brain temperature is higher than the temperature of the brain's blood supply, an in each case brain temperature increases before aortal blood temperature or muscle temperature increases. Thus emotional and drug-induced arousal can cause metabolically expensive neuronal activity that is capable of elevating brain temperature into the range of fever. Regional temperature measurements may offer an index of local metabolic activity that parallels some of the indices drawn from brain imaging studies.